1. Field of the Invention
The present invention relates to a superconducting wire employing an oxide superconductor and a manufacturing method for the same.
2. Description of the Related Art
Oxide superconductors of Y base, Bi base, etc. exhibit superconductivity at a temperature above the boiling point of liquid nitrogen. According to a typical method for forming such a material into a wire, a superconducting material or a material therefor is charged in a metal pipe and subjected to wire drawing, then the charged material is subjected to heat treatment before and after the wire drawing as necessary. As an alternative, an oxide superconductor is formed on a substrate by various thin film forming means such as sputtering. The method wherein a superconductor is charged in a metal pipe has been disclosed in Japanese Patent Laid-Open No. 2-37623 and Japanese Patent Laid-Open No. 1-276516. The method which employs a thin film has been disclosed in Japanese Patent Laid-Open No. 63-241826.
These methods, however, have the following problems: the superconductivity of an oxide superconductor varies depending on the amount of oxygen in the materials; therefore, the amount of oxygen in the materials must be controlled to produce the wire from the superconductor. A superconducting wire is usually provided with a stabilizing material which is normally a metal such as copper. In the case of an oxide superconductor, however, copper is oxidized during the manufacturing process due to the oxygen which is present in the superconductor; therefore, copper cannot be used for the stabilizing material. Further, an oxide superconductor does not have the workability that metals have, preventing crystal grains from deforming easily. This sometimes causes a metal pipe filled with the superconductor material to break during a rolling process or a stretching process which employs dies. In addition, the oxide superconductor must be densely and uniformly charged in the metal pipe to produce usable superconducting wire. There is still another problem: the coefficient of thermal expansion of metal is different from that of an oxide superconductor, presenting a serious problem of adhesion between the metal and the oxide superconductor when they are cooled.
To solve the problems stated above, according to Japanese Patent Laid-Open No. 2-37623, an aluminum pipe is filled with an oxide superconductor and the aluminum is melted and removed before they are heated to sinter the superconductor, then they are subjected to heat treatment at 900 to 1000 degrees centigrade with the oxide superconductor exposed so as to control the amount of oxygen in the material. According to Japanese Patent Laid-Open No. 1-276516, a compact of an oxide superconductor is inserted in a silver pipe and silver powder is charged in the gap between the silver pipe and the superconductor to secure the adhesion between the metal pipe and the superconductor.
According to the method disclosed in Japanese Patent Laid-Open No. 2-37623, however, the melting point of aluminum is approximately 660 degrees centigrade and therefore, the aluminum is very likely to be oxidized by the oxygen present in the oxide superconductor before the aluminum is removed from the surface of the oxide superconductor. It is especially difficult to remove the aluminum which is in a recessed spot of the surface or in a grain boundary of the oxide superconductor; an aluminum oxide generated by oxidation tends to precipitate as an impurity or to react with the oxide superconductor in some cases. Further, the Japanese Patent Laid-Open No. 2-37623 has not disclosed anything about the formation of a stabilizing material which is indispensable for the application to superconductive magnet or the like. According to the method disclosed in the Japanese Patent Laid-Open No. 1-276516, no device has been made to improve the critical current of superconducting wire although the presence of silver powder seems to improve the adhesion between the metal pipe and the oxide superconductor.
According to the method disclosed in the Japanese Patent Laid-Open No. 63-241826 which employs the thin film forming process, the wire is produced beforehand and a thin film composed of the elements for superconducting materials is formed on a substrate, which has copper or a copper alloy deposited on the surface thereof, prior to the heat treatment. It has been disclosed, however, that the heat treatment must be carried out at 800 to 1000 degrees centigrade for 1 to 100 hours, depending on the type of superconducting material used. The method requiring such an extended heat treatment presents a problem of extremely slow manufacture, whereas quicker manufacture of superconducting wire is generally desirable. Moreover, the thin film producing method requires precise control of the composition of the elements constituting the superconductor; a slight change in the composition results in a significant change in the superconductive characteristics, posing a fatal problem in that a lengthy superconducting wire cannot be produced.
Thus, although much study has been done on the manufacture of superconducting wire which employs an oxide superconductor, no superconducting wire for practical use has been available yet.
Accordingly, it is an object of the present invention to provide a practical superconducting wire which uses an oxide superconductor with high critical temperature to prevent the critical temperature and the critical current from dropping during the manufacturing process and a manufacturing method for the same.
To this end, according to one aspect of the present invention, there is provided a superconducting wire having a fine line made of an oxide superconductor which has a metal material dispersed therein, the outer periphery thereof being coated with a conductive material.
According to another aspect of the present invention, there is provided a manufacturing method for a superconducting wire which has a fine line made of an oxide superconductor, comprising: a process for forming a fine line by stretching a metal pipe filled with an oxide superconductor; and a process for heating the fine line at a temperature which is higher than the melting point of the metal material constituting the metal pipe.
According to still another aspect of the present invention, there is provided a manufacturing method for a superconducting wire which has a fine line made of an oxide superconductor, comprising: a process for forming a fine line by stretching a metal pipe filled with a material for an oxide superconductor; a process for reacting the material so as to produce an oxide superconductor, and a process for heating the metal pipe at a temperature which is higher than the melting point of the metal material constituting the metal pipe.